There is a growing trend in the automotive industry to use a single drive belt with which to drive all the accessories run from the engine, rather than a series of separate belts and pulleys. There are definite advantages to the single belt use. Perhaps the single most important advantage is increased service life of such drive belts. Additional advantages include weight reduction, reduced parasitic loss, simplified assembly procedures, higher belt stability, and reduced engine length. Such drive belts are wound around a plurality of pulleys on various accessories, and are therefore referred to as "serpentine" drive belts. The main cause of serpentine drive belt failure is material degradation due to heat caused by belt-slip which slip is, in turn, due to insufficient tension on the belt. This insufficient tension arises from the fact that a single, longer belt is subject to greater stretch or lengthening over its expected use life. This stretch is generally compensated for by an automatic belt tensioner.
Automatic tensioners, in general, comprise an arm with a belt-engaging pulley pivoted to one end thereof and with another end pivoted to the engine block. A yieldable tensioning means maintains a tension on the arm in the direction of belt stretch so that the pulley is continuously and yieldably engaged into the belt to move therewith as the belt stretches. It is desirable that such a yieldable tensioning means maintain a continuous or constant tension on the belt. In addition, the belt is sometimes subject to excessive oscillations or belt "rumble" which may arise from a high air conditioning compressor discharge pressure. It is desirable that the automatic tensioner have a means for damping oscillations induced by the belt on the tensioner arm.
There are several types of automatic belt tensioners which find reflection in patents. One common type of tensioner uses, as the tensioning means, a coil spring wound generally concentrically about the axis at which the arm of the tensioner is pivoted to the engine block, as seen in United Kingdom patent application GB No. 2,102,097A. The coil spring is twisted more tightly when the tensioner is in the minimum take-up position with the belt at its shortest length. The spring unwinds and decreases in tension as the arm rotates and the pulley moves with the belt to the maximum take-up position. There is no way to change the effective point of the application of the coil spring force on the tensioner arm as it moves, and it is consequently difficult to maintain a constant torque about the pivot axis.
Another automatic belt tensioner, shown in U.S. Pat. No. No. 4,351,636, to Hagger uses a straight coil spring which is initially arched, and then straightens as it is squeezed between two abutment members which either approach or move apart from each other as the tensioner arm moves back and forth with the belt. The arching and straightening of the spring are purported to change the spring rate to thereby maintain a constant belt tension. Separate vibration damping pads are provided for belt oscillation damping.
The U.S. Pat. No. 4,270,906, to Kraft et al. uses a stack of Belleville washers which compress a pair of cam plates together to translate the axial force of the Belleville washers into a rotary motion of a tensioner arm. This requires a relatively large axial space to accommodate the Belleville washers and camming plates. No particular structure is disclosed for damping belt oscillations.